Well-defined hetero-interfaces with controlled properties are crucial for any high-performance, semiconductor-based, (opto-)electronic device. They are particularly important for device structures on the nanoscale with increased interfacial areas. Utilizing a ultrahigh-vacuum based multi-tip scanning tunneling microscope, this work reveals inadvertent conductivity channels between the nanowire (NW) base and the substrate, when measuring individual vertical core-shell III-V-semiconductor NWs. For that, four-terminal probing is applied on freestanding, epitaxially grown coaxial p-GaAs/i-GaInP/n-GaInP NWs without the need of nanoscale lithography or deposition of electrical contacts. This advanced analysis, carried out after composition-selective wet chemical etching, reveals a substantially degraded electrical performance of the freestanding NWs compared to detached ones. In an electron beam induced current mode of the nanosensor, charge separation at the substrate-to-NW base junction is demonstrated. An energy dispersive X-ray spectroscopic linescan shows an unintended compositional change of the epitaxially grown NW toward the planar layers caused by different incorporation mechanisms of Ga and In at the NW base. This approach provides direct insight into the NW-substrate transition area and leads to a model of the conductivity channels at the NW base, which should, in principle, be considered in the fabrication of all NW heterostructures grown bottom-up on heterogeneous substrate materials. © 2022 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH.

We report on the design, fabrication, and characterization of a monolithically integrated epitaxial thin-film resistor for THz applications. The device is made of an n+-InGaAs layer grown lattice-matched on an insulating InP:Fe substrate. In this study, on-wafer vector scattering parameter characterization up to 500 GHz is carried out, the calibration is performed by means of the multiline TRL method. The InGaAs thin-film resistor shows very flat frequency response in the range from 100 GHz to 500 GHz. An experiment designed to investigate the resistor's temperature dependence and stability up to 120 °C has also been carried out. © 2022 IEEE.

THz oscillators with on-chip antennas containing no ground plane are affected by substrate modes and therefore undirected radiation into free space. When integrating such antennas with focusing lenses, accurate sub-μm alignment is required. This work presents an assembly process utilizing an HRFZ-Si transfer substrate between the InP RTD chip and a hyper-hemispherical silicon lens, enabling precise alignment. We developed a bonding process for 1.2 × 1.2 mm2 InP RTD chips with an optical adhesive utilizing an advanced sub-micron bonder. THz-TDS measurements of the HRFZ silicon wafer were carried out to analyze the losses within the created setup by EM simulations. The assembly process was verified with measurements of a 300 GHz triple barrier (TB)-RTD oscillator using an SBD detector. © 2022 IEEE.

A broadband THz detector consisting of a triple-barrier InP Resonant Tunneling Diode (RTD) with a monolithically integrated circularly polarized spiral antenna is designed, fabricated, and measured at room temperature. A free space measurement setup is utilized for far-field characterization. The detector (evaluated at zero-bias) is illuminated by a chopped continuous wave signal in the 220-330 GHz band, and the direct detection scheme consists of a lock-in amplifier in voltage mode readout. The measured average responsivity RV is in the range of 750 V/W with a peak of 900 V/W at 257.5 GHz, with the lowest calculated NEP of 2.5 pW/√Hz. © 2021 IEEE.

This paper presents an on-chip 1 to 4 subarray element for wireless subharmonic injection (WSI) in the context of antenna-in-antenna THz oscillators. The proposed antenna receives the third-order subharmonic injection signal (SIS) at 100 GHz from one side and radiates the 300 GHz fundamental oscillation signal (FOS) to the opposite side, which performs like a subharmonic transmitarray. Each element is consisted of a single SIS receiving antenna (Receiver antenna, RA) connected with a 2×2 FOS array (Transmitter antenna, TA). By positioning more FOS antenna around the single SIS antenna, the element spacing at 300 GHz is shorted within one wavelength which inhibits the grating lobe. Through tuning the distance of the FOS array element, the surface wave in the thick indium phosphide (InP) substrate is also reduced to some degree. The simulation results show that the maximum radiation efficiency of the designed chip antenna structure is better than 50% in both the 100 GHz and the 300 GHz band. The conjugate impedance matching in the dual-band is achieved according to the active element requirement. Utilizing the antenna proposed in this work, a low injection loss is verified in the periodical boundary based WSI simulation. © 2021 IEEE.

In this paper, we present a comprehensive study of position-defined Al-polar AlN nucleation on lithographically patterned Si(1 1 1) substrates as a method to obtain ordered Ga-polar GaN nanowire arrays, with possible application in future nanowire-based devices such as LEDs and photoelectrochemical water-splitting cells. In a hydrogen processing step, ex situ prepared oxide on pre-structured Si-pillars could be selectively removed. This enabled Al-polar AlN nucleation on the Si-pillar's sidewalls during the following metal–organic vapor phase epitaxy, while dominant N-polar AlN layer growth was observed on the still oxidized Si(1 1 1) horizontal substrate surface neighboring the pillars. 100% of the Ga-polar GaN wires are emerging on the Al-polar AlN growth sites, thus selective area epitaxy without any mask material could be realized. To gain a precise understanding of the growth mechanisms, the attainable Ga-adatom collection area per NW was varied by changing the Si-pillars’ placement pattern. The wire length and diameter increase with extended pitch. At a constant pitch, the size of the wires is adjustable by variation of the Si-pillars’ diameter, therefore growth of GaN wires of controllable dimension and pitch could be attained. Additionally, parasitic NW growth was completely suppressed for any pitch < 3.5 µm, while an increased pitch resulted in additional parasitic growth. Based on these results a model was derived, which includes the site-controlled removal of the oxide, the thus achieved local polarity control of AlN growth, and the influence of the collection area of each NW with respect to their size, whereas the collection area could be set in the experiment by adjustment of the lithographically controlled pitch and growth-parameter dependent Ga-adatom diffusion length. The mask-less polarity- and site-controlled growth of NWs with a height of 5.3 µm ± 0,33 µm and a diameter of 800 nm ± 160 nm at a pitch of 2.5 µm could be attained. Hence, a deep understanding of the growth mechanisms and the geometrical control of polarity- and site-controlled GaN NWs could be achieved, forming the base for development of NW-based devices on a conductive AlN/Si-template. © 2021

The traditional transmission coefficient present in the original Landauer formulation, which is valid for quasi-static scenarios with working frequencies below the inverse of the electron transit time, is substituted by a novel time-dependent displacement current coefficient valid for frequencies above this limit. Our model captures in a simple way the displacement current component of the total current, which at frequencies larger than the inverse of the electron transit time can be more relevant than the particle component. The proposed model is applied to compute the response of a resonant tunneling diode from 10 GHz up to 5 THz. We show that tunneling electron devices are intrinsically nonlinear at such high frequencies, even under small-signal conditions, due to memory effects related to the displacement current. We show that these intrinsic nonlinearities (anharmonicities) represent an advantage, rather than a drawback, as they open the path for tunneling devices in many THz applications, and avoid further device downscaling. © 1980-2012 IEEE.

On-chip antennas with radiation towards the substrate are affected by modest coupling performance to a free-space path. (Hyper-)hemispherical silicon lenses can improve the efficiency of quasi-optical emission and detection even at THz frequencies. This approach requires an alignment accuracy in the $\mu\mathrm{m}$-scale at THz frequencies. In this contribution, we report on the benefit of hyper-hemispherical silicon lenses in terms of relaxed alignment accuracy needs. We present the impact of alignment on quasi-optical measurements using indium phosphide resonant-tunneling diodes. The main components of the resulting setups are discussed while the effect of alignment is quantitatively evaluated for both, hemispherical and hyper-hemispherical silicon lenses. Moreover, design rules and concepts for a heterointegrated system are derived on consecutive observations. © 2021 IEEE.

Herein, a detailed analysis of leakage mechanisms in epitaxially grown nanowire heterojunction bipolar transistors (NW-HBTs) is presented. Coaxial npn-GaAs/InGaP core–multishell nanowires are grown via gold-catalyzed metalorganic vapor phase epitaxy, processed to three terminal devices and electrically characterized. The key for successful NW-HBT device functionality is the identification of tunneling as the dominant leakage mechanism in highly doped nanowire pn-junctions. The suppression of forward tunneling currents by adjustment of the tunneling barrier width reduces the junction leakage current density into the nA cm−2 regime, which is further verified by tunneling-related electroluminescence measurements. In addition, the suppressed tunneling accordingly increases the number of electrons that are injected from the n-emitter into the p-base. The latter effect influences the performance of pn-junction based devices and is found to enable bipolar transistor functionality. Measured common emitter Gummel plots of the NW-HBT exhibit a current gain of up to 9 and the transistor function is additionally verified by current-controlled output characteristics. © 2020 The Authors. Published by Wiley-VCH GmbH

Metal organic vapor-phase epitaxy of GaN shells on N- and Ga-polar nanowires on AlN/Si(111) templates has been studied in detail. A polarity-dependent epitaxial optimization of nitride-based core-shell structures is necessary to attain the desired shell shape. On N-polar wires, a maximal shell length has been achieved using N2, only, as a carrier gas, while the length decreases by substitution of N2 with H2. A strong impact of the NW growth template polarity has been observed, which has to be considered to attain the desired shell shape. On Ga-polar wires under pure N2, an exclusive coverage of the wire tip occurs. Shell growth and an increasing shell length are obtained by injecting increased H2 flows. The semi-polar {1011} and polar (0001) planes have been identified as the facets that limit the vertical shell length growth evolution on the N- and Ga-polar core-shell structures, respectively. Meanwhile, the m-planar lateral growth mode is found to be identical for both types of polarities. The data are used to set up a growth model that includes the facet-dependent termination, carrier-gas dependent H-passivation, Ga-adatom length and Ga-adlayer formation, and the thereby adjusted three-dimensional growth and shell shape for both polarities. The attained insights and the developed technology allow the epitaxy of homogeneous complex crystal architectures, mandatory for optimized nitride core-shell NW-based devices. This journal is © The Royal Society of Chemistry.

Nanowire chip-based electrical and optical devices such as biochemical sensors, physical detectors, or light emitters combine outstanding functionality with a small footprint, reducing expensive material and energy consumption. The core functionality of many nanowire-based devices is embedded in their p-n junctions. To fully unleash their potential, such nanowire-based devices require – besides a high performance – stability and reliability. Here, we report on an axial p-n junction GaAs nanowire X-ray detector that enables ultra-high spatial resolution (~200 nm) compared to micron scale conventional ones. In-operando X-ray analytical techniques based on a focused synchrotron X-ray nanobeam allow probing the internal electrical field and observing hot electron effects at the nanoscale. Finally, we study device stability and find a selective hot electron induced oxidization in the n-doped segment of the p-n junction. Our findings demonstrate capabilities and limitations of p-n junction nanowires, providing insight for further improvement and eventual integration into on-chip devices. © 2020, The Author(s).

A large-signal equivalent circuit model is developed for ultra-high frequency signal generation and detection provided by an InP triple barrier resonant tunneling diode. On-wafer DC and S-parameter measurements on 0.5 um2 and 1 um2 area devices were made from 20 MHz to 67 GHz. The bias dependent measurement data are utilized to extract the parameters of a compact RF model which accurately describes the static and dynamic behavior of the triple barrier resonant tunneling under zero bias and forward bias condition. © 2020 IEEE.

Herein, the characterization of n-doped InGaP:Si shells in coaxial not-intentionally doped (nid)-GaAs/n-InGaP as well as n–p–n core–multishell nanowires grown by metalorganic vapor-phase epitaxy is reported. The multi-tip scanning tunneling microscopy technique is used for contact-independent resistance profiling along the tapered nid-GaAs/n-InGaP core–shell nanowires to estimate the established emitter shell doping concentration to ND ≈ 3 · 1018 cm−3. Contacts on these shells are demonstrated and exhibit ohmic current–voltage characteristics after annealing. Application potential is demonstrated by the growth and processing of coaxial p-GaAs/n-InGaP junctions in n–p–n core–multishell nanowires, with n-InGaP being the electron-supplying emitter material. Current–voltage characteristics and temperature-dependent electroluminescence measurements substantiate successful doping of the n-InGaP shell. A tunneling-assisted contribution to the leakage currents of the investigated p–n junctions is verified by the sub-bandgap luminescence at low temperatures and is attributed to radiative tunneling processes. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We present Au catalyzed p-GaAs nanowire growth on n-GaN layers as a possible method to grow an arsenide on a nitride compound semiconductor by metal organic vapor phase epitaxy. The GaAs growth position, the nanowire density and the nanowire growth direction are controlled by a combination of vapor-liquid-solid growth and selective area epitaxy. Thus, a spatially controlled nanowire growth is attained, which is mandatory for device fabrication. The growth position is defined by lithographically positioned Au discs on n-GaN. By adapting the growth conditions (QTBAs, presaturation) the nanowire density is optimized. Lateral and vertical anisotropic nanowire growth is attained through VLS growth in structured SiOx openings. Critical technological parameters for successful control of the growth direction are the positioning of the Au catalyst in relation to the SiOx mask, the size of the eutectic in relation to the opening dimensions, and the SiOx thickness. These results lead to distinct pn-junction positions and adjustable nanowire growth dimensions and directions. This journal is © The Royal Society of Chemistry.

Phase and frequency control of resonant tunneling diode (RTD) based terahertz oscillators are major challenges in realizing coherent signal sources for arrayed applications, such as spatial power combining, beam steering, or multi-in multi-out systems. In this letter, we demonstrate frequency locking and control of an RTD oscillating at f0 ∼ 550 GHz, via radiative injection of a weak sinusoidal subharmonic signal at f0/2. Precise frequency control, within the locking range of around 2 GHz, is demonstrated. A peak output power enhancement of 14 dB in the whole locking range, compared to the free running oscillator, is achieved. Furthermore, occurrence of phase locking is identified by the spectral linewidth reduction, quantifiable in the full-width at half-maximum parameter. A signal linewidth of 490 Hz was achieved in locked operation. © 2011-2012 IEEE.

Modeling of quantum devices is still based on the original idea of Landauer that the macroscopic (DC) conductance of electron devices can be related to the (microscopic) transmission coefficient of electrons. In this paper we propose a simple model that captures the role of the particle and displacement currents in quantum electron devices working at THz by substituting the traditional transmission coefficient by a new displacement current coefficient. In particular, our model is used to compute the total current of a Resonant Tunnelling Diode (RTD) device under AC conditions. The new model, based on a time-dependent approach, is firstly shown to reproduce the DC behaviour of the Landauer model. Later on, at input frequencies higher than 500 GHz, large differences between the two models are observed. In particular, unexpected high frequency behavior is observed in simulations with an input signal up to 2 THz. © 2020 IEEE.

This paper discusses advances related to the integration of future mobile electronic THz systems. Without claiming to provide a comprehensive review of this surging research area, the authors gathered research on selected topics that are expected to be of relevance for the future exploration of components for practical mobile THz imaging and sensing applications. First, a brief technology review of integrated mobile THz components is given. Advances in III-V technology, silicon technology, and resonant-tunneling diodes (RTD) are discussed. Based on an RTD source and a SiGe-HBT direct detector, low-cost and compact computed tomography is presented for volumetric continuous-wave imaging at around 300 GHz. Moreover, aspects of system integration of mobile THz MIMO radars are discussed. Thereby, a novel phase-locked loop concept utilizing a high-stability yttrium-iron-garnet-tuned oscillator to synthesize ultra-stable reference mmWave signals is shown, and an adaptive self-interference cancellation algorithm for THz MIMO in the digital domain based on Kalman filter theory is proposed. © 2020, Springer Science+Business Media, LLC, part of Springer Nature.

The monolithic on-chip integration and design of a high current density InP-based Triple Barrier Resonant Tunneling Diode within a bow-tie antenna structure for detection application is presented. The asymmetrical current-voltage characteristics of the Triple Barrier Resonant Tunneling Diode and its small capacitance provide a powerful candidate for THz signal detection. The integration into a planar broadband antenna structure such as a bow-tie design enables realization of a broadband detector from single GHz up to THz frequencies. In this work, experimental data are demonstrated in the frequency range of 75 to 110 GHz and from 220 to 330 GHz. © 2019 IEEE

We report a method for analyzing and evaluating quantum transport parameters on the basis of our theoretical approach and experimental data in triple-barrier resonant tunneling diodes. Effective tunneling time and phase relaxation time are extracted from measured S-parameters with determination of parasitic components. © 2019 IEEE.

The charge transport through GaAs nanowires, partially p-doped and partially intrinsic, is analyzed by four-point resistance profiling along freestanding nanowires using a multip-STM. The charge transport channel in the undoped segment is assigned to the surface conductivity, while the interior of the nanowire is the conductance channel in the p-doped segment. The convoluted interplay between conduction through the interior of the nanowire and surface state conduction is studied in detail. Measurements of the I-V curves along the nanowires provide the experimental basis for the proposed charge transport model for the transition of the conduction from the interior to the surface of the nanowire. A voltage drop along the surface state conduction channel leads to an upward shift of the band edges at the surface. This results, for higher applied voltages, in the removal of the depletion layer and an opening of a conductance channel between the interior of the nanowire and the surface states. © 2019 IOP Publishing Ltd.

We present a novel approach to attain Ga-polar n-GaN nanowires on n-Si(111)/AlN templates, by site- and polarity-controlled metal organic vapor phase epitaxy. A three-stage process is developed to (i) form equally-sized Ga-polar GaN islands, (ii) change the growth direction towards the vertical direction and finally, to (iii) obtain continuous nanowire epitaxy. Homogeneous islands are achieved by minimizing parasitic nucleation and adjusting the adatom diffusion length to the used nanoimprint pattern. The influence of the carrier gas composition on the polarity is studied, achieving pure Ga-polarity by mostly using nitrogen carrier gas. Enhancing the Si/Ga-ratio leads to an amplification of the vertical growth, but also to a reduced number of NWs. 100% growth is attained by a height dependent V/III-ratio adjustment. The results are supported by a qualitative model, explaining how suppression of multi-pod, parasitic and inhomogeneous crystallization can be realized by trading off in situ SiNx passivation and localized GaN growth. © 2019 The Royal Society of Chemistry.

The chemisorption of various functionalizing agents on (100) and (111)B gallium arsenide and (100) indium phosphide substrates is studied to elaborate a wet-chemical surface coating protocol for gallium arsenide based nanowires. Application of (non-)fluorinated alkanethiols under different parameters shows great success resulting in a decreased polarity of the surface (confirmed by a significant increase of the water contact angle) and no occurrence of etching effects. The successful functionalization is determined by X-ray photoelectron spectroscopy measurements. After optimization of concentration, additives and solvents, the process parameters are transferred to both, p- and n-doped GaAs-based nanowire structures. The influence of surface functionalization on the electrical behavior of the nanowires is determined by current–voltage characteristics. Based on the experimental data, a bonding mechanism for the alkanethiol onto the semiconductor material is proposed and a model for describing surface depletion before and after functionalization is developed. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In this work the reduction of reverse currents in Au-catalyzed, MOVPE grown coaxial GaAs nanowire diodes are reported. The reduction is achieved by introducing an interstitial, lattice-matched i-InGaP shell (spacer) as tunneling barrier inside the junction, which also functions as a selective etch stop. With increasing spacer thickness, rectification ratios of &gt;1.57 × 106 at ± 1.65 V, ideality factors of 1.3, and dark saturation current densities as low as 20 pA cm−2 are extracted, which are related to a reduced tunneling probability. Temperature-dependent DC measurements of junctions with thin spacers show a correlation to a simple (trap-assisted) tunneling model. With absolute reverse currents in the pA range down to −3 V bias, the improved diode is implemented as a collector-base junction in a coaxial n(i)pn nanowire structure by growing an additional, outer n-doped InGaP shell as the emitter layer in a nanowire HBT. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In this paper, a transmitarray element (TE) is designed for wireless subharmonic injection-locked triple barrier (TB) resonant tunneling diode (RTD) oscillators. It adopts a receiver antenna (RA)-transmitter antenna (TA) structure. The RA is a u-slotted patch antenna, and we use a cubic silicon block at top of this patch, so as to increases the RA gain and radiation efficiency. A fat monopole structure with a slot-like counterpoise is used as the TA. In this design, the RA can receive 100 GHz subharmonic injection signal (SIS). Meanwhile, the TA will radiate the 300 GHz fundamental oscillation signal (FOS) generated by the TB RTD. Moreover, the TA structure can isolate the 300 GHz FOS coming into the RA but couple the received 100 GHz SIS to the TB RTD, which performs like a filter-antenna. In the simulation, the transmission loss in the TA structure is higher than 15 dB around 300 GHz and only about 1.5 dB around 100 GHz. The gain of RA is 6 dBi with 65% radiation efficiency at 100 GHz and the gain of TA is 14 dBi at 300 GHz when applying a 1 mm radius silicon lens at the backside of the InP substrate. © 2019 IEEE.

Signal detection at (sub-)mm-wave frequencies via a single chip size component is discussed. The monolithic integration consists of high current density InP-based Triple Barrier Resonant Tunneling Diode into an on-chip antenna. The asymmetrical current voltage characteristic of the Triple Barrier Resonant Tunneling Diode enables signal detection at zero bias. A very high responsivity above 250 GHz is experimentally demonstrated. Low temperature DC rectification factor of the diode is investigated and a thermionic current contribution over the temperature is presented. © 2019 IMA - Institut fur Mikrowellen- und Antennentechnik e.V.

In this paper we present a millimeter-wave detector based on a triple-barrier resonant tunneling diode and a broadband Archimedean spiral antenna (ASA). Due to the conjugate matching between them, the power transfer from the receiving antenna to the rectifying diode is maximized. Besides the broadband impedance and radiation characteristics of the ASA, the circular-polarization is an essential property for mobile applications. The detector functionality has been evaluated around 200 GHz by non-linear circuit simulation, where the antenna coupling to an incoming arbitrary polarized wave is incorporated via a three-port touchstone file that has been obtained before by full-wave FEM simulation. © 2017 IEEE.

Metal organic vapor phase epitaxy is used to grow gallium arsenide (GaAs) nanocrystals (NCs) on germanium (Ge) templates on nanoscopic silicon (Si) threads prepared by reactive ion etching. Scanning transmission electron microscopy with energy dispersive X-ray measurements shows an epitaxial growth of the GaAs on the Ge template that is supported by the Si thread, and that Ge doping is induced to the GaAs by the template. On Ge templates of about 60 nm diameter, as-grown GaAs NCs show a very regular rhombic-dodecahedral outer shape that can be explained by a preferential growth along the <110> plane. Photoluminescence measurements of the Ge/GaAs structures reveal radiative emission peaks on top of the GaAs band-to-band emission and at sub-band gap energies. While high energy peaks are originating from Ge acceptor levels in GaAs, sub-band gap peaks can be explained by radiation from Ge donor and acceptor bands that are amplified by photonic modes hosted in the rhombic-dodecahedral GaAs NCs. This study shows that a template-assisted crystal growth at the nanoscale opens up routes for a versatile integration of strongly emitting nanomaterials for a use in on-chip solid state lighting and photonics. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Profiling of the electrical properties of nanowires (NWs) and NW heterocontacts with high spatial resolution is a challenge for any application and advanced NW device development. For appropriate NW analysis, we have established a four-point prober, which is combined in vacuo with a state-of-the-art vapor-liquid-solid preparation, enabling contamination-free NW characterization with high spatial resolution. With this ultrahigh-vacuum-based multi-tip scanning tunneling microscopy (MT-STM), we obtained the resistance and doping profiles of freestanding NWs, along with surface-sensitive information. Our in-system 4-probe STM approach decreased the detection limit for low dopant concentrations to the depleted case in upright standing NWs, while increasing the spatial resolution and considering radial depletion regions, which may originate from surface changes. Accordingly, the surface potential of oxide-free GaAs NW {112} facets has been estimated to be lower than 20 mV, indicating a NW surface with very low surface state density. [Figure not available: see fulltext.]. © 2018, Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature.

Signal generation and detection at mm-wave frequencies via a single chip size component is demonstrated. The monolithic integration consists of high current density Triple Barrier Resonant Tunneling Diode into a slot antenna. The asymmetrical current voltage characteristic of the Triple Barrier Resonant Tunneling Diode enables signal detection at zero bias and signal generation at forward bias within the regime of negative resistance. Signal generation and detection at above 250 GHz are experimentally demonstrated. © 2018 IEEE.

A site- and polarity-controlled MOVPE growth of 3D-GaN on Si(111) substrates is established using the polarity-dependent growth speed of GaN on an intermediate AlN layer. For hydrogenated Si or elevated AlN growth temperatures mixed-polar growth is observed. N-polarity could be realized on oxidized Si(111) surfaces by a reduced AlN growth temperature of TAlN = 930 °C. Specific Si crystal facets (e.g., {100}, {112}) are determined as starting points for metal-polar growth of AlN. By site-controlled etching in Si, we intentionally expose these additional crystal facets prior to epitaxy to obtain defined starting points for metal-polar growth. At TAlN = 930 °C, this leads to a site-controlled growth of metal-polar GaN, surrounded by N-polar AlN on Si(111). Thus both, a polarity- and site-controlled epitaxial growth of 3D-GaN is achieved. The new developed method has been applied to a large variety of structure sizes from 650 nm to 1 mm. The results are supported by a schematic model, explaining the influence of surface termination, in situ desorption, and growth conditions. This paves the way to the development of future 3D-opto-electronic devices, directly established on Si. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Wire-less sub-harmonic injection locking of a free running oscillator is presented. The free running oscillator consists of an InP-based triple barrier resonant tunneling diode integrated in a slot antenna designed for oscillation at f0 from 31 GHz to 35 GHz. The wire-less sub-harmonic injection locking signal is provided by a horn antenna to the slot antenna resonator of the fundamental mode RTD oscillator. The injection signal is set to a frequency of fsub = f0/2. A frequency locking and control with a precise factor of 2 is experimentally demonstrated which is a proof of concept for wire-less sub harmonic injection locking of a free running oscillator. © 2017 IEEE.

Spin-momentum locking in a semiconductor device with strong spin-orbit coupling (SOC) is thought to be an important prerequisite for the formation of Majorana bound states. Such a helical state is predicted in one-dimensional (1D) nanowires subject to strong Rashba SOC and spin-mixing - its hallmark being a characteristic re-entrant behaviour in the conductance. Here, we report direct experimental observations of the re-entrant conductance feature, which reveals the formation of a helical liquid, in the lowest 1D subband of an InAs nanowire. Surprisingly, the feature is very prominent also in the absence of magnetic fields. This behaviour suggests that exchange interactions have a substantial impact on transport in our device. We attribute the opening of the pseudogap to spin-flipping two-particle backscattering. The all-electric origin of the ideal helical transport could have important implications for topological quantum computing. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

We report on a prototype device geometry where a number of quantum point contacts are connected in series in a single quasi-ballistic InAs nanowire. At finite magnetic field the backscattering length is increased up to the micron-scale and the quantum point contacts are connected adiabatically. Hence, several input gates can control the outcome of a ballistic logic operation. The absence of backscattering is explained in terms of selective population of spatially separated edge channels. Evidence is provided by regular Aharonov-Bohm-type conductance oscillations in transverse magnetic fields, in agreement with magnetoconductance calculations. The observation of the Shubnikov-de Haas effect at large magnetic fields corroborates the existence of spatially separated edge channels and provides a new means for nanowire characterization. © 2016 American Chemical Society.

The transport in InAs nanowires is investigated at low temperatures. On wires with different n-type doping information on Rashba and Dresselhaus spin-orbit coupling is gained from weak antilocalization measurements. By using a short local gate quantum point contacts are formed, which show quantized conductance. From bias-depended measurements the g-factor is extracted for different subbands. © 2016 IEEE.

One-dimensional ballistic transport is demonstrated for a high-mobility InAs nanowire device. Unlike conventional quantum point contacts (QPCs) created in a two-dimensional electron gas, the nanowire QPCs represent one-dimensional constrictions formed inside a quasi-one-dimensional conductor. For each QPC, the local subband occupation can be controlled individually between zero and up to six degenerate modes. At large out-of-plane magnetic fields Landau quantization and Zeeman splitting emerge and comprehensive voltage bias spectroscopy is performed. Confinement-induced quenching of the orbital motion gives rise to significantly modified subband-dependent Landé g factors. A pronounced g factor enhancement related to Coulomb exchange interaction is reported. Many-body effects of that kind also manifest in the observation of the 0.7·2e2/h conductance anomaly, commonly found in planar devices. © 2016 American Chemical Society.

Scanning electron microscope (SEM) induced nanowire (NW) attraction or bundling is a well known effect, which is mainly ascribed to structural or material dependent properties. However, there have also been recent reports of electron beam induced nanowire bending by SEM imaging, which is not fully explained by the current models, especially when considering the electro-dynamic interaction between NWs. In this article, we contribute to the understanding of this phenomenon, by introducing an electro-dynamic model based on capacitor and Lorentz force interaction, where the active NW bending is stimulated by an electromagnetic force between individual wires. The model includes geometrical, electrical, and mechanical NW parameters, as well as the influence of the electron beam source parameters and is validated using in-situ observations of electron beam induced GaAs nanowire (NW) bending by SEM imaging. © 2016 AIP Publishing LLC.

Nanowire growth on heteroepitaxial GaP/Si(111) by metalorganic vapor phase epitaxy requires the [-1-1-1] face, i.e., GaP(111) material with B-type polarity. Low-energy electron diffraction (LEED) allows us to identify the polarity of GaP grown on Si(111), since (2×2) and (1×1) surface reconstructions are associated with GaP(111)A and GaP(111)B, respectively. In dependence on the pre-growth treatment of the Si(111) substrates, we were able to control the polarity of the GaP buffers. GaP films grown on the H-terminated Si(111) surface exhibited A-type polarity, while GaP grown on Si surfaces terminated with arsenic exhibited a (1×1) LEED pattern, indicating B-type polarity. We obtained vertical GaAs nanowire growth on heteroepitaxial GaP with (1×1) surface reconstruction only, in agreement with growth experiments on homoepitaxially grown GaP(111). © 2015 AIP Publishing LLC.

The transport data of n-doped gallium-nitride self-assembled nanowires grown by metal-organic vapor-phase-epitaxy are determined. The wire diameter varies from 0.4 μm to 1.6 μm while the length was up to 50 μm. Optical lithography and lift-off were used to form Ti/Au multiple contacts to the nanowires for transmission line measurements. A specific contact resistance of ρ<inf>C</inf> = 1.74 × 10-7 Ω cm2 and a nanowire resistivity of ρ<inf>NW</inf> = 2.27 × 10-3 Ω cm could be determined. Electrical conductivity measurements were carried out and a model of the wire resistance as a function of the nanowire radius and the charge carrier concentration was developed. Using this model, the magnitude of the doping level of the n-GaN nanowires is determined. Based on this data, a dopant concentration of the GaN wires of about n = 1020cm-3 has been investigated. © 2015 AIP Publishing LLC.

(Graph Presented). The high speed on-off performance of GaN-based light-emitting diodes (LEDs) grown in c-plane direction is limited by long carrier lifetimes caused by spontaneous and piezoelectric polarization. This work demonstrates that this limitation can be overcome by m-planar core-shell InGaN/GaN nanowire LEDs grown on Si(111). Time-resolved electroluminescence studies exhibit 90-10% rise- and fall-times of about 220 ps under GHz electrical excitation. The data underline the potential of these devices for optical data communication in polymer fibers and free space. © 2015 American Chemical Society.

GaN-on-Si transistors attract increasing interest for power applications. However, the breakdown behavior of such devices remains below theoretical expectations, for which the Si substrate is typically made responsible. In this work, the effect of the thickness of an aluminum nitride buffer layer on the vertical breakdown voltage, measured relative to a grounded silicon substrate, has been investigated. A voltage-polarity-dependent breakdown mechanism has been observed. It has been found that the breakdown in the positive bias voltage regime is initiated by carrier injection, for which the carriers originate from an inversion channel formed between the epitaxial layers and the p-silicon substrate. TCAD simulations have confirmed the proposed explanations, and suggest that appropriate modification of the electronic structure at the AlN/silicon interface could significantly improve the vertical breakdown voltage. © 2014 IOP Publishing Ltd.

InP-based resonant tunneling diodes with symmetrical I/V-characteristics have shown their excellent high frequency performance for THz signal generation. For signal detection we present a device with an additional third barrier to create an unsymmetrical I/V-characteristic. Sensitivity measurements are performed and further improvements by scaling of the active device area are discussed. To allow SPICE based circuit simulation an approach for a large signal model is presented. © 2013 IEEE.

Nanowire FETs with all-around Junction-Gate are demonstrated using GaAs core-shell nanowires. The electrical properties of the n-channel Junction FET were determined by DC measurements. The radial pn-junctions show diode-type I-V characteristics. The output and transfer I-V characteristics exhibit good pinch-off, and hysteresis-free transient behavior. First devices with 190 nm nanowire channel diameter show a drain current of ID = 260 nA and a transconductance of gm = 300 nS. © 2013 IEEE.

The recombination dynamics of vapor-liquid-solid grown GaAs-nanowires with an axial p-n heterojunction is investigated by spatially and time-resolved photoluminescence spectroscopy. By scanning across the doping transition of single p-n and n-p doped nanowires, respectively, the particular influence of surface losses in differently doped areas is studied. We found a significantly reduced non-radiative recombination for the n-doped region compared to the p-doped one, which can be attributed to suppressed surface losses because of the characteristic band bending at the surface. © 2013 AIP Publishing LLC.

Resistance profiles along as-grown GaAs nanowires were measured with a multi-tip scanning tunneling microscope used as a nanoprober. The nanowires were grown in the vapor-liquid-solid growth mode in a two-temperature-step mode and doped with Zn. Using a transport model, the resistance profile was converted to a dopant profile. The dopant distribution along the nanowires was found to correlate with the temperature during different phases of nanowire growth. The nanowire base grown at higher temperature exhibits a decreased dopant concentration. Mechanical stress by intentional bending of a nanowire was shown not to influence nanowire conductance. © 2013 AIP Publishing LLC.

We have fabricated a spin transistor based on InAs nanowire. The transistor operates based on Datta-Das type spin transistor mode [1][2][3]. The spin polarized electrons are injected from the ferromagnetic electrodes and synchronized spin precession is controlled by the gate voltage through spin-orbit interaction. We have clearly observed drain current oscillation versus gate voltage as expected from the Dyakonov-Pérel mechanism. By controlling the spin precession, on/off switching is expected to be achieved with the steep slope. The best of our obtained results has shown the steepest slope of 18mV/dec to 23mV/dec near the off-state ((2n+1)π-spin rotation) where vertical electric field meats the condition of persistent spin helix (PSH) motion [3]. The present result provide alternative method of steep slope device mechanism in addition to the conventional ideas such as Tunnel FETs or impact ionization FETs. © 2012 IEEE.

Recently, heavy emitter doping rather than decreasing the barrier thickness has boosted the peak current density of resonant tunneling diodes (RTDs) above 1,000 kA/cm 2. Based on this achievement very mature InP-based RTD with current densities above 500 kA/cm 2 are nowadays the leading solid-state THz device [1, 2]. Here, we show that even triple-barrier RTD (TBRTD) devices now reach a current density in excess of 250 kA/cm 2 making this element ideally suited for rectification [3] but now at THz frequencies. Figure 1 is the state of art of THz detection sensitivity of previously reported zero bias detectors. Focusing on such zero bias broadband THz detection, we have also been studying on a design policy for a μm-sized on-chip self-complementally antenna and especially we have reported basic performances of a bow-tie antenna[4,5] integrated with a conventional homogeneous semiconductor mesa structure. However, it was still limited studies considering neither of actual nonlinear devices and peripheral circuits. © 2012 IEEE.

The conductivity and crystal structure of nominally undoped InAs nanowires deposited by three different methods - 1. selective area metal organic vapor phase epitaxy (SA MOVPE), 2. gold assisted vapor liquid solid (VLS) MOVPE and 3. extrinsic catalyst free VLS molecular beam epitaxy (MBE) - is investigated. The influence on conductivity by stacking faults and different growth conditions is analyzed to determine the main impact. It is found that in terms of crystal structure, nanowires deposited by VLS MOVPE and VLS MBE behave similarly showing a zinc blende (ZB) phase while nanowires deposited by SA MOVPE feature a high density of stacking faults and a tendency to higher amounts of wurtzite (WZ) when grown with a decreased growth rate. However, the conductivity of wires deposited by VLS MOVPE is found to be much higher and statistically less dispersive compared to the other two wire types. An electrical similarity between nominally undoped wires in VLS MOVPE and previously reported intentionally doped wires in SA MOVPE is observed and discussed. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Axial GaAs nanowire p-n diodes, possibly one of the core elements of future nanowire solar cells and light emitters, were grown via the Au-assisted vapor-liquid-solid mode, contacted by electron beam lithography, and investigated using electron beam induced current measurements. The minority carrier diffusion lengths and dynamics of both, electrons and holes, were determined directly at the vicinity of the p-n junction. The generated photocurrent shows an exponential decay on both sides of the junction and the extracted diffusion lengths are about 1 order of magnitude lower compared to bulk material due to surface recombination. Moreover, the observed strong diameter-dependence is well in line with the surface-to-volume ratio of semiconductor nanowires. Estimating the surface recombination velocities clearly indicates a nonabrupt p-n junction, which is in essential agreement with the model of delayed dopant incorporation in the Au-assisted vapor-liquid-solid mechanism. Surface passivation using ammonium sulfide effectively reduces the surface recombination and thus leads to higher minority carrier diffusion lengths. © 2012 American Chemical Society.

The optical and electrical characterization of nanostructures is crucial for all applications in nanophotonics. Particularly important is the knowledge of the optical near-field distribution for the design of future photonic devices. A common method to determine optical near-fields is scanning near-field optical microscopy (SNOM) which is slow and might distort the near-field. Here, we present a technique that permits sensing indirectly the infrared near-field in GaAs nanowires via its second-harmonic generated (SHG) signal utilizing a nonscanning far-field microscope. Using an incident light of 820 nm and the very short mean free path (16 nm) of the SHG signal in GaAs, we demonstrate a fast surface sensitive imaging technique without using a SNOM. We observe periodic intensity patterns in untapered and tapered GaAs nanowires that are attributed to the fundamental mode of a guided wave modulating the Mie-scattered incident light. The periodicity of the interferences permits to accurately determine the nanowires radii by just using optical microscopy, i.e., without requiring electron microscopy. © 2012 American Chemical Society.

III/V semiconductor nanowires are grown by the vapour-liquid solid growth mode from Au seed particles in an industrial type metal-organic vapour phase epitaxial apparatus. For electronic applications InAs nanowires with very high electron were developed on InAs (111), InAs (100), and GaAs (111) substrates. The wireswere deposited on insulating host substrate for metal-insulator-semiconductor FET fabrication. Their excellent DC and RF performance are presented. For optoelectronic applications the focus is on selective n- and p-type doping. GaAs nanowires with an axial p-n junction are presented. Pronounced electroluminescence at room temperature reveals the quality of the fabricated device. Moreover, spatially resolved photocurrent microscopy shows that optical generation of carriers took place only in the vicinity of the p-n junction. A solar conversion efficiency of 9% was obtained. In summary, III/V semiconductor nanowires are emerged to high performance and versatile nanoscaled building blocks for both electronic and optoelectronic applications. © Springer-Verlag Berlin Heidelberg 2012.

We use the nonlinear optical property of GaAs to directly visualize the path of the near infrared incident laser light coupled into individual nanowires. We fully illuminate with near infrared pulse laser untapered and tapered GaAs nanowires grown via the Au-assisted vapor-liquid-solid mechanism. We record second-harmonic generation (SHG) signals in the visible spectrum. In some nanowires, an interference pattern is observed and investigated in terms of distances between the maxima of the SHG signal taking into account the effective refractive index in such sub wavelength structures with radius below 90 nm. We propose a model to explain the periodicity of the maxima in the SHG interference pattern. The theoretical model includes the waveguiding and the Mie scattering theories for obtaining the 2π periodicity fitting well the experiments. Moreover, we also measure interferences in tapererd nanowires with a radius down to 76 nm. The possible effect of the gold in non radiative recombination and the presence of the gold particle at the tip of some nanowires are also discussed. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

We report on the fabrication and analysis of basic digital circuits containing InAs nanowire transistors on a host substrate. The nanowires were assembled at predefined positions by means of electric field-assisted self-assembly within each run generating numerous circuits simultaneously. Inverter circuits composed of two separated nanowire transistors forming a driver and an active load have been fabricated. The inverter circuits exhibit a gain (>1) in the MHz regime and a time constant of about 0.9 ns. A sample & hold core element is fabricated based on an InAs nanowire transistor connected to a hold capacitor, both on a Silicon and an InP isolating substrate, respectively. The low leakage read-out of the hold capacitor is done by InP-based metal-insulator heterojunction FET grown on the same substrate prior to nanowire FET fabrication. Experimental operation of the circuit is demonstrated at 100 MHz sampling frequency. The presented approach enables III/V high-speed, low-voltage logic circuits on a wide variety of host substrates which may be up scaled to high volume circuits. Copyright © 2012 The Institute of Electronics, Information and Communication Engineers.

Heterostructure n-GaAs/InGaP/p-GaAs core-multishell nanowire diodes are synthesized by metal-organic vapor-phase epitaxy. This structure allows a reproducible, selective wet etching of the individual shells and therefore a simplified contacting of single nanowire p-i-n junctions. Nanowire diodes show leakage currents in a low pA range and at a high rectification ratio of 3500 (at ±1V). Pronounced electroluminescence at 1.4 eV is measured at room temperature and gives evidence of the device quality. Photocurrent generation is demonstrated at the complete area of the nanowire p-i-n junction by scanning photocurrent microscopy. A solar-conversion efficiency of 4.7%, an open-circuit voltage of 0.5 V and a fill factor of 52% are obtained under AM 1.5G conditions. These results will guide the development of nanowire-based photonic and photovoltaic devices. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this letter the scalability of electrical properties of axial GaAs nanowire pn-diodes grown by Au-assisted metalorganic vapor-phase epitaxy is reported. The impact of the nanowire diameter on the forward current, the ideality factor, and the diode series resistance were investigated. Electrical measurements carried out on various single nanowires reveal the existence of a critical diameter for device functionality. Below that diameter the diode series resistance increases infinitely, which can be attributed to surface depletion. Above that diameter the forward current increases linearly with the junction area, while the series resistance demonstrates inverse proportionality. Hereby the ideality factor remains approximately constant at a value of 2. In addition, the analyzed axial GaAs nanowire pn-diodes offer very high rectification ratios ranging from 10 4 to 10 6 at ±2 V. © 2011 TMS.

InP-based resonant tunneling diodes with symmetrical I/V-characteristics have shown their excellent high frequency performance for THz signal generation. We present a modification with an additional third barrier to create an unsymmetrical I/V-characteristic. With their large current densities and low capacitances these devices are promising candidates for zero bias high frequency envelope detectors. Based on simulations two layer stacks are grown by MBE technology. The fabricated devices were measured at dc- and high frequencies. First measurement results for the short circuit responsivity are discussed. © 2012 IEEE.

GaAs nanowire pn-diodes were used for single nanowire radial and axial photovoltaic device fabrication. In this study the performance under very high illumination power is measured and analyzed in terms open circuit voltage, and efficiency. It is found that GaAs nanowire photovoltaic devices can be operated under very high power of up to 120 W/cm2 (λ = 532 nm) without detectable degradation. The short circuit current increases directly proportional to the illumination power while the open circuit voltage steadily increases up to the highest illumination. This study shows that both axial and radial nanowire pn-junctions are suitable for sun light conversion with very high concentration ratio ( &gt; 1,000). © 2012 IEEE.

Electrical and optical properties of semiconducting nanowires (NWs) strongly depend on their diameters. Therefore, a precise knowledge of their diameters is essential for any kind of device integration. Here, we present an optical method based on dark field optical microscopy to easily determine the diameters of individual NWs with an accuracy of a few nanometers and thus a relative error of less than 10%. The underlying physical principle of this method is that strong Mie resonances dominate the optical scattering spectra of most semiconducting NWs and can thus be exploited. The feasibility of this method is demonstrated using GaAs NWs but it should be applicable to most types of semiconducting NWs as well. Dark field optical microscopy shows that even slight tapering of the NWs, i.e.diameter variations of a few nanometers, can be detected by a visible color change. Abrupt diameter changes of a few nanometers, as they occur for example when growth conditions vary, can be determined as well. In addition a profound analysis of the elastic scattering properties of individual GaAs NWs is presented theoretically using Mie calculations as well as experimentally by dark field microscopy. This method has the advantage that no vacuum technique is needed, a fast and reliable analysis is possible based on cheap standard hardware. © 2011 IOP Publishing Ltd.

We report on axial pn-junctions in GaAs nanowires. The nanowires were grown by MOVPE on (1 1 1)B GaAs substrates using the vaporliquidsolid mechanism in combination with Au seed particles. At the low growth temperature of 400 °C any additional growth on the nanowire sidewalls can be excluded such that a pure axial pn-junction is realized. p-Type doping was provided by diethyl zinc, while tetraethyl tin was introduced for n-type doping. The impact of dopant supply was investigated both on structural properties and on carrier density. The carrier type was independently verified by processed nanowire metalinsulator FETs. The lengths of the whole pn-GaAs nanowires reach up to 20 μm while their diameters are up to a few 100 nm, as defined by the Au seed particles used. The pn-GaAs nanowire device exhibits diode-like IV characteristics and strong electroluminescence. While the reverse current is in the low pA-regime, the forward current reaches a few μA, limited by the n-doped side. A diffusion voltage VD=1.4 V is determined, which corresponds to the GaAs band gap energy. To our knowledge this is the first axial GaAs pn-diode realized in a single GaAs nanowire. © 2010 Elsevier B.V.

Our study shows the impact of the process parameters V/III ratio, pressure and temperature on growth and morphology of GaN nanowires (NWs) synthesized by an Au-initiated vapour-liquid-solid mechanism on a sapphire substrate. We confined a temperature window for successful GaN NW growth and show how the variation of reactor pressure changes the NW morphology. Using a very low V/III ratio, NW tapering, which was observed for higher V/III ratios, could be avoided. The optimization of these process parameters led to non-tapered GaN NWs, aligned perpendicular to the substrate. Further evaluation by scanning electron microscopy showed a high density (~3·109/cm2) of hexagonal c-plane GaN NWs having diameters of 60 ± 9 nm. Transmission electron microscopy revealed single-crystalline NWs without threading dislocations but some stacking faults. The use of a very low V/III ratio was found to be important for the successful selective growth and, most interestingly, led to a difference in NW and gold catalyst droplet diameter. For chemical analysis of the NW and its catalyst droplet, electron energy loss spectroscopy was employed confirming gold as the catalyst material. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report on a simple Inductively Coupled Plasma-Reactive Ion Etching (ICP-RIE) process with Cl2/N2 chemistry to process InP based, self-aligned HBTs with sub-micron emitters. Since the layer to be etched is in the range of 150 nm (the thickness of emitter cap and emitter layers), a low etch rate is beneficial. On the other hand, it is also necessary to use chemistries without hydrogen to prevent any possible hydrogen passivation. Therefore, in this work, Cl2/N2 chemistry is selected and a plasma process providing an etch rate of 120 nm/min is optimized. Not only the etch rate but also the electrical and the surface quality of the wafers are examined. It has been illustrated that the etch rate of the optimized process is uniform over the wafer and it is reproducible. In addition to that, it has been shown with electrical measurements that there is no degradation in the material quality. To test the optimized process, sub-micron HBTs are fabricated and the RF measurements have shown an fmax of 340 GHz which make them to be used in high speed communication systems. In addition to that, lower and controlled under etch gives better current gain distribution over the wafer leading better device models and resulting in better yield in MMICs. © 2011 Elsevier B.V. All rights reserved.

Semiconductor nanowires may be used as the channel of a field-effect transistor device. In contrast to field-effect transistors made of epitaxial layers, the nanowire approach provides large material diversity and enables a fully surrounding gate contact. In this way, no carriers can escape from the channel and a higher transconductance is routinely observed. In contrast to carbon nanotubes the charge polarity can be selected and a metallic phase that may inhibit the channel depletion is avoided. In this chapter device concepts will be presented based on semiconductor band structure and heterostructures. Recent advances in device technology and selfassembly will be discussed. The corresponding DC performance of the different nanowire approaches is reviewed. Special emphasis is given on the measurement and the performance of nanowire field-effect transistors at high frequencies. © 2011 Bentham Science Publishers Ltd. All rights reserved.

In this letter, n-type doping of GaAs nanowires grown by metal-organic vapor phase epitaxy in the vapor-liquid-solid growth mode on (111)B GaAs substrates is reported. A low growth temperature of 400°C is adjusted in order to exclude shell growth. The impact of doping precursors on the morphology of GaAs nanowires was investigated. Tetraethyl tin as doping precursor enables heavily n-type doped GaAs nanowires in a relatively small process window while no doping effect could be found for ditertiarybutylsilane. Electrical measurements carried out on single nanowires reveal an axially non-uniform doping profile. Within a number of wires from the same run, the donor concentrations N D of GaAs nanowires are found to vary from 7 × 10 17 cm -3 to 2 × 10 18 cm -3. The n-type conductivity is proven by the transfer characteristics of fabricated nanowire metal-insulator-semiconductor field-effect transistor devices. © 2010 The Author(s).

We report on the technology and the electrical properties of two different contact systems on n-GaAs nanowires. Annealed Ge/Ni/Ge/Au and Pd/Ge/Au multilayer metallization were investigated. Rapid thermal annealing at temperatures common for identical contact systems on n-GaAs layers is found to be crucial due to an enhanced out-diffusion of the Ga component into the Au contact layer. The maximum annealing temperatures ensuring intact nanowires are 320°C for Ge/Ni/Ge/Au and 280 °C for Pd/Ge/Au. The fabricated Pd/Ge/Au contacts reveal a specific contacts resistance of 2.77×10-7 ωcm2, which is about one order of magnitude lower compared to the values of Ge/Ni/Ge/Au and also lower than Pd/Ge/Au contacts on bulk material (1.2 ×10-6 ωcm2). © 2011 American Institute of Physics.

We present GaAs electroluminescent nanowire structures fabricated by metal organic vapor phase epitaxy. Electroluminescent structures were realized in both axial pn-junctions in single GaAs nanowires and free-standing nanowire arrays with a pn-junction formed between nanowires and substrate, respectively. The electroluminescence emission peak from single nanowire pn-junctions at 10 K was registered at an energy of around 1.32 eV and shifted to 1.4 eV with an increasing current. The line is attributed to the recombination in the compensated region present in the nanowire due to the memory effect of the vapor-liquid-solid growth mechanism. Arrayed nanowire electroluminescent structures with a pn-junction formed between nanowires and substrate demonstrated at 5 K a strong electroluminescence peak at 1.488 eV and two shoulder peaks at 1.455 and 1.519 eV. The main emission line was attributed to the recombination in the p-doped GaAs. The other two lines correspond to the tunneling-assisted photon emission and band-edge recombination in the abrupt junction, respectively. Electroluminescence spectra are compared with the micro-photoluminescence spectra taken along the single p-, n-and single nanowire pn-junctions to find the origin of the electroluminescence peaks, the distribution of doping species and the sharpness of the junctions. © 2011 IOP Publishing Ltd.

We report on detailed structural and morphological characterizations of InAs nanowires of 〈001〉, 〈111〉, and 〈112〉 crystallographic directions grown on (001)B InAs wafer substrates using high-resolution transmission electron microscopy. We find that 〈001〉 -oriented InAs nanowires are cubic zincblende-type structure and free of planar defects. The 〈111〉- and 〈112〉-oriented InAs nanowires both have densely twinned (111) planar defects that are perpendicular and parallel to the growth direction, respectively. The cross sections of all three types of InAs nanowires are obtained from 3D reconstructions using electron tomography. The characteristics of the planar defects and the 3D wire shape should provide better estimations of microstructure-relevant physical properties, such as conductivity and Young's modulus of InAs nanowires. © 2011 American Institute of Physics.

The spatially resolved photoelectric response of a single axial GaAs nanowire pn-diode has been investigated with scanning photocurrent and Kelvin probe force microscopy. Optical generation of carriers at the pn-junction has been shown to dominate the photoresponse. A photocurrent of 88 pA, an open circuit voltage of 0.56 V and a fill factor of 69% were obtained under AM 1.5 G conditions. The photocurrent followed the increasing photoexcitation with 0.24 A/W up to an illumination density of at least 90 W/cm2, which is important for potential applications in concentrator solar cells. © 2011 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

Nanowires allow for a combination of highly mismatched materials needed for optimized broad spectrum absorption [1], and the use of appropriate substrates. III-V nanowires offer in addition a high absorption coefficient which is indispensable if a high efficiency energy transformation is aimed at the nanoscale. Currently, both radial [2] as well as axial III-V nanowire pn-junctions are under discussion for photovoltaic applications. The axial device enables a high forward current, ultra-low leakage, and a possible staggered integration of multiple cells. Moreover, the axial device gives full access for a high resolution photovoltaic analysis that is especially important at the current status of evaluation since a detailed device understanding is pending and the published yield data of nanowire devices are still low [1-2]. © 2011 IEEE.

Nanowires can excellently be controlled during synthesis with respect to physical and chemical characteristics, including composition, size, electronic and optical properties. They may be used both as devices and interconnects, and thus can open doors for downscaled integration concepts not seen before. The non-lithographic bottom up synthesis approach on the nanoscale may be extremely cost-effective, especially when making use of the large material diversity stemming from decoupling of device from substrate material without loss of structural quality, e.g. growing metallic, Ge or III-V nanowires on Si substrates. Going down to very small dimensions one may make use of quantum confinement effects like reduced phonon scattering and related high carrier mobility, tunable electrical and optical properties, or implementing heterostructures for quantum dot and single electron devices. © 2010 IEEE.

In this paper, a 50-μ m-pitch coplanar waveguide pattern for on-wafer high-frequency measurements on nanowire FET is used. The contact structure exhibits relatively large parasitic elements in comparison to the intrinsic device making a precise deembedding both necessary and challenging. A single InAs nanowire FET with a large gate length of 1.4 μm possesses after deembedding a maximum stable gain higher than 30 dB and a maximum oscillation frequency of 15 GHz. The gate length scaling of the nanowire transistor is modeled using the experimental transconductance data of a set of transistors and an analytical model. On this basis, both the device performance and the expectation of high-frequency measurements at small gate lengths are discussed. © 2006 IEEE.

A unipolar n-n heterostrucuture diode is developed in the InP material system. The electronic barrier is formed by a saw tooth type of conduction band bending which consists of a quaternary In-0.52(AlyGa1-y)(0.48)As layer with 0 < y < y(max). This barrier is lattice matched for all y to InP and is embedded between two n(+)-InGaAs layers. By varying the maximum Al-content from y(max,1) = 0.7 to y(max,2) = 1 a variable barrier height is formed which enables a diode-type I-V characteristic by epitaxial design with an adjustable current density within 3 orders of magnitude. The high current density of the diode with the lower barrier height (y(max,1) = 0.7) makes it suitable for high frequency applications at low signal levels. RF measurements reveal a speed index of 52 ps/V at V-D = 0.15 V. The device is investigated for RF-to-DC power conversion in UHF RFID transponders with low-amplitude RF signals.

InP-based double-barrier resonant tunnelling diodes have been optimized for high speed digital circuits. We present the scalability of high current density (JP ≈ 150 kA/cm2) resonant tunnelling diodes in the sub-micrometer electrode area range. A small signal equivalent circuit has been developed and its parameters are precisely deduced from DC and RF measurements. Based on this model the scalability has been investigated with emphasis on a low but also scalable series resistance in order to keep the peak voltage constant. A comparison of dry and wet etching methods in the device fabrication will be presented. A multiple mesa concept has been adopted to provide reliable scalability at low emitter area (AE < 1 μm2).

This review demonstrates that ion irradiation is a very useful tool in order to tailor the properties of semiconductor nanowires. Besides optical and electrical doping provided by adequate ion species and ion energies, one can use ion beams also for the controlled shaping of the morphology of nanostructures. Here, one utilizes the commonly as 'negative' described characteristics of ion implantation: defect formation and sputtering. We show that ion beams can be even used for an alignment of the nanowires. Furthermore, we report here on several successful experiments in order to modify the electrical and optical properties in a controlled manner of ZnO semiconductor nanowires by the use of transition metals, rare earth elements and hydrogen ions. Schematic illustration of ion beam doping of a single contacted nanowire. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.